New glassy materials for optics,
optoelectronics and light fiber technique
Jan WasylakUniversity o f Mining and Metallurgy, al. Adama Micldewicza 30, 30 — 059 Kraków, Poland.
The results of the investigation o f new kinds o f amorphous glassy materials, the methods o f their preparation, and their properties with a view o f applying them in modern optics are presented. As a result of a change in the chemical composition and the applications of special synthesis techniques, it is possible to change the optical, spectral, technological, and other properties of glass. The oxide, halide, and chalcogenide-halide glasses were used for investigation purposes. Owing to the properties o f the material it is possible to obtain optical fibers characterized by a wide transmission range, which can be made use of in optical applications.
1. Introduction
Due to their physicochemical properties, amorphous glassy materials are finding more and more application in modern science and technology.
An important progress has recently been made in the preparation of a new composition of oxide, halide, chalcogenide, and mixed glasses, and optical properties of these unconventional glasses have been discovered. The possibility of shifting the transmittance edge in the IR towards longer wavelengths and, as a consequence, attaining transmittance of the middle IR are the chief reasons for this investigation.
It is possible to obtain the infrared transmitting glasses using the heavy cations due to the low frequency of fundamental vibrations of relatively weak cation — anion bond. Lead and bismuth oxides can be introduced into the slicate or borate glasses in large amounts, nevertheless it is the fundamental glass matrix which is responsible for the limitation of the infrared light transmittance.
Recently, there has been observed a growing interest in the application of oxygen-free glasses as materials for IR optics, non-linear optics, and light pipe techniques. Investigations into new compositions of these glasses concentrate on shifting the absorption edge of the glasses in the IR towards longer wavelengths. The prospective materials for IR and non-linear optics are the chalcogenide glasses. Transmission of these glasses in the IR may reach a value as high as some tens of micrometers, whereas in oxide glasses the value attained is of the order of 8.5 pm. The glassy state and certain properties of the chalcogenide systems have been the subject of investigations carried out by many authors [1] — [5],
6 J. Wasylak
Glasses obtained in these systems are based chiefly on sulfur components. The synthesis of these glasses requires some specific method of preparing the batch and of melting and cooling the melt.
The interest in glasses showing the preceding optical properties has directed the authors’ attention to the possible synthesis of mixed halide — chalcogenide glasses. Our earlier investigations of halide — chalcogenide glasses were carried out based on the chalcogenides with halides of univalent cations. Reports on the new halide
— chalcogenide glasses with the heavy metals have been published in [6] —[8]. A system was selected for investigation in which suflur was replaced by selenium, which is characterized by considerably higher transmittance in the IR (up to 18 pm) when compared with sulfur (11 pm) as well as by its more advantageous effect on the optical properties of the glasses [9].
In this paper, attention is focused on a limited number of stable glasses and their optical properties.
2. Experimental method
The glasses examined were melted in an electric furnace: the lead —bismuth — phosphate ones in alumina covered crucibles at 1100—1300 °C, the lead — bismuth —gallium ones in platinum covered crucibles at 1050—1250 DC. The molten glass was poured into a iron mould and annealed.
The fluoride glasses, chalcogenide glasses and halide — chalcogenide glasses were melted in glassy coal with a cover. The crucible was next placed in an electric furnace in argon atmosphere. The glasses were melted at the temperature of about 660 — 680 °C for 30 minutes. After the crucible was taken out from the furnace the alloy was poured into a precooled metal form and annealed.
Determination of the glassy area was performd by means of X-ray diffraction analysis, and DTA examination was also carried out in order to determine the characteristic temperatures as well as the thermal stability. Such properties of the glasses as density, refractive index, light transmittance and microhardness have also been determined.
3. Investigation results
3.1. Lead — bismuth — phosphate oxide glasses
The glassy state area in the B a(P03)2 — Bi20 3 — PbO system is shown in Fig. 1, and the glass compositions are presented in Tab. 1. The glassy state area in this system is limited by partial surface and volumetric crystallization.
The glassy stability was estimated on the basis of the characteristic temperatures, the criterion being the difference between the crystallization onset temperature Tx and the transition temperature Tg. This difference varies depending on the glass composition attaining maximum for the most stable glass (Tab. 2).
The values of density, microhardness and refractive index of lead —bismuth — phosphate glasses are given in Tab. 3.
B a(P 0 3)2
Fig. 1. Glassy state area in PbO —Bi20 3 —B a (P 0 3)2 system: · — homogeneous glass, x — partial crystallization.
T a b l e 1. Composition of lead-bismuth-phosphate glasses selected for investigations
Component (mol%) 1 Sample number 2 3 4 5 PbO 18.75 17.00 1250 8.33 4.14 B i j 0 3 26.28 23.00 17.50 11.67 5.83 B a (P 0 3)2 55.00 60.00 70.00 80.00 90.00
T a b l e 2. Characteristic temperatures of lead—bismuth -phosphate glasses
Sample Characteristic temperatures [°C] T, TA X Tc Tr AT 1 615 640 685 840 820 875 25 2 550 590 650 695 735 800 830 875 40 3 565 610 700 735 785 875 55 4 520 570 670 700 815 850 50 5 525 540 675 730 770 790 15
The light transmittance of these glasses covers the range 0.25 — 5 pm (Fig. 2). The absorption band at about 3 pm results from the presence of the O H - groups in the glasses.
3.2. Lead — bismuth — gallium oxide glasses
The glassy state area in the PbO —Bi20 3 —G a20 3 system (Fig. 3) is limited by crystallization and sinters.
8 J. Wasylak
A [fjm ]
Fig. 2. Light transmittance of lead — bismuth — phosphate glasses (denotations as for Tab. 1).
T a b l e 3. Values of the density, microhardness and refractive index for lead-bismuth-phosphate glasses
Sample d [g/cm 3] H V [G Pa] n 1 5.71 1.15 1.69 2 5.37 1.12 1.68 3 4.87 1.11 1.67 4 4.68 1.32 1.65 5 3.64 1.32 1.64
The shapes of the DTA curves and the characteristic temperatures of the glasses are much diversified and strongly dependent on the glass composition. The characteristic temperatures for several glasses are given in Tab. 4.
T a b l e 4. Characteristic temperatures of the lead —bismuth —gallium glasses
PbO Bi20 3 G a20 3 Characteristic temperature [°C] glass composition [mol%] l \ T. Tc Tf 6 0 - 5 - 3 0 420 440 500 525 615 3 0 - 4 0 - 3 0 365 405 450 485 540 560 7 5 - 0 - 2 5 — — 490 525 595 740 6 0 - 1 5 - 2 5 355 380 470 535 585 595 3 0 - 4 5 - 2 5 355 370 435 510 (560) 590 610 2 0 - 5 5 - 2 5 345 370 (420) 470 495 580 710 5 5 - 2 5 - 2 0 325 350 425 525 570 615 40 - 40 - 20 325 350 440 465 555 610 40 - 4 5 - 1 5 305 340 390 450 485 565 600 3 0 - 5 5 - 1 5 305 335 385 410 490 550 590
Ga20 3
Fig. 3. Glassy state area in PbO —Bi20 3 —G a20 3 system.
T a b l e 5. Values of the density, microhardness and refractive index for lead —bismuth —gallium glasses
P bO —Bi20 3—G a20 3 d [g/cm 3] H V [G Pa] n
6 5 - 5 - 3 0 7.35 3.04 2.24 6 0 - 1 0 - 3 0 7.53 3.11 22 6 5 0 - 2 0 - 3 0 7.61 3.31 2 2 8 4 0 - 3 0 - 3 0 7.79 3.43 2.30 3 0 - 4 0 - 3 0 7.87 3.45 2.35 7 5 - 0 - 2 5 7.57 2.91 2.30 7 0 - 5 - 2 5 7.63 2.81 2.33 6 0 - 1 5 - 2 5 7.72 2.97 2.37 4 0 - 3 5 - 2 5 7.98 3.00 2.40 2 0 - 5 5 - 2 5 8.15 3.21 2.43 1 5 - 6 0 - 2 5 8.19 3.26 2.45 5 5 - 2 5 - 2 0 7.99 2.77 2.43 4 0 - 4 0 - 20 8.21 3.04 2.45 2 5 - 5 5 - 2 0 8.29 3.16 2.51 5 0 - 3 5 - 1 5 8.34 2.80 2.48 4 0 - 4 5 - 1 5 8.39 2.87 2.49 3 0 - 5 5 - 1 5 8.45 2.96 2.54
The values of density, microhardness and refractive index for lead—bismuth — gallium glasses are given in Tab. 5. High values of density and refractive index of the glasses are associated with the presence of the heavy metal cations. These quantities decrease with increasing gallium oxide content.
The light transmittance of these glasses covers the range 0.5 —8.3 pm (Fig. 4). The absorption band at about 3.1 pm is connected with the presence of the OH~ groups in the glasses. The light transmittance decreases with increasing content of the heavy metal oxide and at the same time the short- and long-wave spectral limits shift towards higher wavelengths.
10 J. Wasylak
A [pm]
Fig. 4. Light transmittance of lead —bismuth—gallium glasses: — — — PbO (25 mol% )—Bi20 3 (50 m ol0/· ) —G a20 3 (25 mol%), --- PbO (40 m ol0/. ) - B i 2O 3 (45 m ol0/. ) - G a 2O 3 (15 m ol0/·). 3 3 . Halide glasses
This term is used to denote glasses in which the electronegative part is formed by halides, such as Cl, Br, I. From the point of view of technology many of these materials are useless because of their high hydroscopicity, low temperature of softening and tendency to crystallize. Intensive research in this field has been carried out with a view to attain greater transmittance in infrared of the chloride and iodide glasses in comparison with the fluoride glasses. The halide glasses were prepared from the halides of zinc, cadmium, bismuth and thorium. ZnCl2 can easily form glass with infrared cut off in the range of 20 pm; however, the glassy ZnBr2 demonstrates very low chemical resistance and great tendency to crystallize. CdCl2 shows a tendency towards vitrification when it occurs together with other halides, such as Pbl2 and PbCl2. In this case the transmittance in infrared attains up to 20 pm.
3.4. Fluoride glasses
Fluoride glasses are characterized by a great tendency to crystallize and they frequently occur in a crystalline state. Only a small number of multicomponent compositions yield optical glasses suitable to form large samples and allowing light fibers to be drawn. A good criterion for the selection of glass is the critical cooling rate Vc, connected with the formation of the first crystals. If the rate of cooling from liquids to solidus is higher than Vc, the glass obtained is free from dispersed crystals.
The best materials have undoubtedly the smallest Vc, about 1 an^ these are glasses (formed) on the basis of ZrF4 [10] —[13]. Glasses in the ZrF4 — BaF2 — LaF3 — A1F3 system have Vc equal to 20 “/min* and the same glass with an addition of NaF shows Vc = 1 °/min. These two glasses, in which small changes in their composition are possible, are the prospective candidates for light fiber drawing. The basic structural units of the fluoride glasses on ZrF4 basis are the octahedrons
The second type of fluoride glasses are glasses on the basis of ThF4 without ZrF4. The third family comprises glasses on the basis of A1F3 which has the property of forming glass with other components, such as BaF2, ThF4 and YF3. The critical cooling rate Vc of these glasses falls within the limit 100 °/min. An addition of In F 3 contributes to the optimization of the composition and reduction of Vc to 5 °/min.
Many interesting optical properties of fluoride glasses are connected with the wide transmission range, region from UV light to middle IR, where the threshold of infrared absorption is situated between 6 and 8 pm, depending on composition.
3.5. Chalcogenide glasses
The absorption edge in chalcogenide glasses is shifted towards longer waves (0.9 — 20 pm, depending on their chemical composition.
The transmission characteristic for chalcogenide glasses as compared with other oxide glasses is shown in Fig. 5 [14].
Fig. 5. Transmission characteristic for: 1 — PbO —G e 0 2 glass, 2 — CaO — A120 3 — S i0 2 glass, 3 — As2S3 glass, 4 — As2 75Te0 25 glass and 5 — A s2Se3 glass.
3.6. Halide — chalcogenide glasses
Promising materials for transmission in infrared are glasses in the HgS —PbBr2 — P b l2 and Sb2S3 —HgS —PbBr2 systems [15]. The transmission of these glasses falls within the interval 0.5 —15 pm. Glasses of this type were synthesized in a glove box at a temperature of about 500 °C, in argon atmosphere, in order to avoid the S 0 2 absorption band.
It is often easier to obtain glass containing both chalcogenides and halides than purely chalcogenide glasses. Sb2S3 is considered to be glass-forming, however obtaining Sb2S3 glass is very difficult as the temperature of its transformation coincides with that of crystallization.
An addition of halides as modifiers allows us to obtain glass in two- and three-component systems. An example here are glasses in the Sb2Se3 —BaCl2
12 J. Wasylak
Sb2Se3
Fig. 6. Glassy state in the Sb2S e3 —BaCl2 —PbCl2 system: the open circles denote glass, and filled circles denote the crystalline phase.
Figure 6 shows the glassy area in the Sb2Se3 —BaCl2 —PbCl2 system.
As is seen from this figure, the glassy area adheres to two-component Sb2Se3 — PbCl2 system in the concentration range of Sb2Se3 from 50 to 80 mol%. The third component, BaCl2, increases the tendency towards glass formation in a two-component system. In the two-component Sb2Se3 —BaCl2 system the difficul ty in obtaining glass is connected with high field intensity of the cation Ba2+.
The transmission range of the obtained glasses falls within the limit from 0.65 to 50 pm, at a level of 38 — 60%, and the refractive index n > 2.5 (Fig. 7).
Fig. 7. Light transmittance o f the glass 80 Sb2Se3 10 BaCl2 PbCl2.
3.7. Possibilities of applying the glasses transmitting infrared radiation
The optical materials used in infrared techniques are expected to meet many requirements. The most important ones are: the maximal transmittance of IR radiation in the given range of spectrum, a definite value of refractive index as a function of wavelength, good mechanical and chemical properties.
Glasses showing high infrared transmittance may be used as optically active materials for light fibers. Recent investigations of light fibers operating in IR region are often carried out with a view to demonstrate the possibility of producing fibers from the given material. However, most of light fibers produced in that way cannot be used in practice on account of the toxicity of the materials, very poor mechanical properties of the fibers, weak chemical resistance, narrow range of the transformation temperatures, sensitiveness to ultraviolet radiation, no possibility to cover it with another material to form the core-coat structure, limitation of the thermal treatment of the material, its susceptibility to crystal lization and poor viscosity in the course of drawing the fiber.
In spite of the existence of the above problems, the IR light fibers find more and more application. At present they may be utilised:
— in medicine (in light fiber-laser surgery, angioplasty, measurement of the content of gases in blood, endoscopy in IR band),
— in industry (systems of automatic control engineering, telespectroscopy of gases and liquids, radiometric temperature measurements, humidity measure ments, measurements in area of increased level of ionizing radiation, e.g., in nuclear power plants, systems of thermovision and thermography, control of polymerization of composite materials),
— in the army (viewfinder systems, teledetection noctovision, telecommunica tion systems with increased tolerance of ionizing radiation, warning systems).
With respect to the type of the light fiber and the function it performs, the above applications can be arranged into five groups:
1) detectors (gauges, sensors) — for spectroscopy, pyrometry, inteferometry, radiometry,
2) fiberscopes (fiber-optic cables) — for shifting the focal plane and fusion of the focal plane,
3) power engineering — transmission of electromagnetic wave of great energy,
4) telecommunication — single-mode light fibers with ultralow losses with compensated dispersion,
5) active and non-linear light fiber elements.
Most studies in the area of IR light fibers are performed on transparent light in a band in which the oxide glasses become opaque or their transparency is greatly reduced, i.e., from about 3 pm. This limit is valid at present, however, in future it may be shifted towards longer waves. The long wave transparency limit depends essentially on the kind of the material used and it may reach 30 — 40 pm.
Glasses characterized by high infrared transmittance are also applied to produce elements of devices used in IR techniques. These are elements which are used in the visible range of the radiation spectrum, i.e., mirrors, prisms, lenses, filters. They differ, however, from the above mentioned glasses in that they are distinguished by a higher value of the refractive index, better mechanical strength, greater resistance to the influence of atmospheric factors, and in the case of filters, in that they do not transmitt visible radiation.
14 J. Wasylak
References
[1] Ling Z., Lin H , Chengshan Z., New chalcogenide glasses from the Sb2S3 — M X m system, [In] Proc. 9th In t Symp. on N on-O xide Glasses, Chin. Ceram. Soc., Zhejiand Univ. The Shang. Ceram. Soc., Hangzhou, China, 1994, pp. 4 4 — 49.
[2] Meresse Y , Fonteneau G., Lucas L, New germanium sulfide based glasses, [In] Proc 9th Int. Symp. on Non-Oxide Glasses, Chin. Ceram. Soc. Zhejiand Univ. The Shang. Ceram Soc. Hangzhou, China, 1994, p. 3 9 5 -3 9 9 .
[3] Wei K., Machewirth D. P., Wenzel J., Snitzer E., Sigel G. H , J. N on-Cryst Sol. 182 (1995), 257. [4] Xiujian Z., Jdciang L , JlANJUN H., Formation and some properties o f the Sb2Se3 — CdBr2
— H gJ2(PbJ2) chalcogenide —halide glasses, [In] Proc. XVI Int. Congress on Glass, Int. Academic
PubL, Beijing, (China), 1955, Vol. 5, p. 402 - 407.
[5] Nageno Y., Takebe H., Morinaga K., Judd — Ofelt parameters in oxide, fluoride and chalcogenide
glasses, [In] Proc. XVII In t Congress on Glass, Chin. Ceram. Soc., In t Academic Publ., Beijing,
(China), 1955, Vol. 5, p. 5 0 5 -5 0 9 .
[6] Robinel E , Carette B., Ribes M., J. Non-Cryst. Solids 57 (1983), 49.
[7] Sun H. W., Tanguy B., Reau J. M , Videau J. I.,Portier J , J. N on-Cryst Solids 99 (1988), 222. [8 ] Zhao X., X u L., Yin H , Sakka S., J. Non-Cryst. Solids 167 (1994), 70.
[9 ] Hilton R., J. Non-Cryst. Solids 2 (1970), 28.
[10] Poulain M , Poulain M., Matecki M., Res. Bull. 16 (1981), 555. [11] Poulain M , Lucas J , Verres Refract. 32 (1978), 505.
[12] Poulain M , Poulain M., J. Non-Cryst Solids 56 (1983), 57. [13] Wasylak J., Samek L , J. Non-Cryst. Solids 129 (1991), 137.
[14] Xiujian Z., jdciang L , Jianjun H., Formation and some properteis o f the Sb2Se2 — CdBr2
—H gJ2(PbJ2) chalcogenide—halide glasses, [In] Proc. XVI Int. Congress on Glass, Int. Academic
Publ., Beijing, (China), 1995, Vol. 5, p. 402 - 407.
[15] Mutz J. L., Poulain M., Chiquet F , Maze G., Influence o f processing and composition on the
properties o f lead chalcogenide glasses, [In] Proc. 8th Symp. on Halide Glasses, CNET,
Perros-Guirec, 1992, p. 166—174.
